The optical time domain reflectometer (OTDR) is an instrument which is used for measuring the attenuation and other characteristics of fibre optic cable. The OTDR sends light pulses into an optical fibre and measures the reflected and backscattered light. The principle advantages of the OTDR method are that the fibre cable does not have to be cut nor do both ends of the fibre cable have to be accessed.
The fibre characteristics and features along the cable will cause portions of the light pulse to be backscattered and reflected back to the OTDR. The OTDR has a photo detector which detects the backscattered and reflected light. The OTDR processes the signals from the photodetector to create a waveform which shows the characteristics and features of the fibre as a function of time. The waveform is then displayed on a monitor as a function of distance along the length of the cable.
The OTDR system can use a range of optical pulse widths to probe and test the fibre cable. In an OTDR system, an optical pulse with a greater width provides superior distance and dynamic range, while an optical pulse with a narrow width provides superior distance resolution. It will be appreciated that a narrow pulse width provides increased resolution for the OTDR at the expense of distance or dynamic range because less energy is sent into and received from in the cable.
The dynamic range and resolution of an OTDR instrument are the two most important specifications. To achieve superior performance, an OTDR must be capable of providing high dynamic range and distance resolution.
The problem is existing OTDR systems is the matching of the receiver frequency response to the varying pulse widths of the transmitter stage. To maximize the efficiency and accuracy of the data acquisition, the receiver must be able to respond to the varying optical pulse widths which can be generated by the transmitter stage. Another problem in existing OTDR systems occurs when a reflected light pulse having a large amplitude (e.g. reflection at connector) saturates the input amplifier and thereby prevents the detection of backscattered light which immediately follows the period of time between saturation and recovery of the amplifier. This is commonly known as the "dead-zone".
Another problem in practical OTDR systems is noise associated with electromagnetic interference. The signals present in the receiver stage of an OTDR are very wide bandwidth and very low level, and therefore the circuit is susceptible to noise which can degrade the dynamic range of the OTDR.